The present invention relates to a heat-resistant sintered hard alloy, composed of a hard phase consisting mainly of a WCoB type complex boride, and a cobalt base alloy matrix phase binding the hard phase which hard alloy exhibits excellent room temperature characteristics as well as excellent high temperature characteristics such as high temperature strength and oxidation resistance, and as a hot extruding die for a copper rod.
Requirements for wear-resistant sintered hard materials have become increasingly severe, and the industry has sought improved materials having wear-resistance as well as heat-resistance and corrosion resistance or the like.
As sintered hard materials, carbides, nitrides and carbonitrides such as WC base hard alloys and TiCN type cermets are well known. As substitute materials for the aforementioned hard materials, hard alloys and cermets including metallic borides such as WB and TiB.sub.2 and metallic complex borides such as Mo.sub.2 FeB.sub.2 and Mo.sub.2 NiB.sub.2 have been recently proposed, noting excellent properties of borides such as extreme hardness, high melting point and high electric conductivity. Further, stellites are utilized as cobalt base wear-resistant materials.
A hard alloy formed by binding WB with a nickel base alloy such as disclosed in Japanese Patent Publications No. Sho 56-45985, No. Sho 56-45986 and No. Sho 56-45987 is a paramagnetic wear-resistant material to be used especially in watch cases and ornaments, and is not intended for structural materials to be used at high temperature.
Ceramics comprising metallic borides such as TiB.sub.2 as disclosed in Japanese Patent Publications No. Sho 61-50909 and No. Sho 63-5353 exhibit extreme hardness and pronounced heat resistance, but impart poor thermal shock resistance due to there being no metallic binding matrix phase.
Generally, hard materials formed by adding metals to metallic borides suffer from the disadvantage in that they tend to form a brittle third phase, and it is difficult to obtain high strength or toughness.
Hard alloys comprising metallic complex borides such as Mo.sub.2 FeB.sub.2 and Mo.sub.2 NiB.sub.2 formed by reaction during sintering have been developed to eliminate the above disadvantage.
A Mo.sub.2 FeB.sub.2 type hard alloy disclosed in Japanese Patent Publication No. Sho 60-57499 has excellent mechanical properties, wear-resistance and corrosion resistance at room temperature but unsatisfactory high temperature strength and oxidation resistance due to its iron base binding matrix phase.
A Mo.sub.2 NiB.sub.2 type hard alloy disclosed in Laid Open Japanese Patent application No. Sho 62-196353 has excellent high temperature properties and corrosion resistance, but poor wear-resistance and anti-adhesion property, since the complex boride Mo.sub.2 NiB.sub.2 is about 15 Gpa at micro-Vickers hardness and is not so hard, and its binding phase consists of nickel base alloy. Stellites exhibit excellent high temperature properties, but their hardness is too low to be used for wear-resistant materials.
It is an object of the present invention to provide a sintered hard alloy having excellent room temperature properties as well as pronounced high temperature properties such as high temperature strength and oxidation resistance.